An unusual DNA polymerase, terminal deoxynucleotidyl transferase, is found exclusively in thymus and thymus-derived cells. It also appears in certain human leukemias and mouse lymphomas. This project will examine the biology of terminal transferase with respect to two questions: 1) Is terminal transferase a marker for a specific thymus-related cell? 2) What is the specific function of this enzyme in such cells? The results should be useful in analyzing the development of some leukemias and perhaps in understanding the development of diversity in the immune system.